The purified transport protein, IIIxt1 of the Xylitol PTS of Lactobacillus casei was apparently isolated in the phosphorylated form (P-IIIxt1). Treatment with alkaline phosphatase markedly reduced the degree of cross-reactivity with antisera prepared against the purified protein. It also produced a change in the electrophoretic mobility of the protein consistent with a loss of hydrophobicity. Thus, the conformational state of this protein can be drastically altered by phosphorylation or dephosphorylation. These obervations will provide a working hypothesis or the mechanism by which IIIxt1 interacts with EIIxt1. Coaggregation of Cytophaga sp. DR2001 and Actinomyces israelii WV0307 is mediated by a protein lectin located on the cell envelope of the former and a complex polysaccharide containing certain-acetylated sugars found the cell wall of the latter. Inhibition studies showed that N-acetyl-galactosamine, n-acetyl-glucosamine and N-acetyl neuraminic acid inhibited coaggregation. When combined in pairs or triads these compounds produced a synergistic effect. The lectin could be demonstrated in membrane preparations of cytophaga cells but not on the vesicles liberated during growth.